The first thing that needs to be worked on is the plot. You need to have a strong plot which can add a lot of intrigue to the party and can make it exciting for the people attending.
Develop characters as per the plot. You will need quite a few characters if you are planning on hosting a big party. In case there is a lack of characters, you will obviously have room for fewer participants.
Develop a theme as per the plot of the party. You can incorporate any kind of appropriate theme for this party that suits the characters and the overall mood of the gathering.
As you develop the characters, it is important to have costumes for each of them. This is important for the party to be lively and give a realistic touch at the same time. Having your characters dressed in period clothing can enhance the mystery and encourage others to get excited about participating.
Invite guests to the event well in advance to make sure that all the main characters are present. Keep a decent entrance fee as you will be spending a fair amount on all the arrangements that are needed for the party. You can use this money to add to your budget for food or minor props that might be needed for staging the mystery.
While you will be a part of the story at the party, be sure to attend to all guests and make sure that they are enjoying themselves. In case that any special arrangements are needed during the event, do help them.
Make sure that along with the intrigue and the mystery, there should be some sort of entertainment available for the guests. Also, make sure that there is ample food available at the venue so that the guests can enjoy themselves.
Have a strong finish to the mystery that adds a bit of a 'wow' factor. Make the end of the story intense and be sure that participants are left with a bit of a surprise with the outcome. This will make the party even more successful.